Conventionally, golf ball covers are made either by compression molding two preformed hemispherical cups about a core or by injection molding thermoplastic cover material about a core. It is standard practice in conventional injection molding to provide a mold having two cavities, each having hemispherical surfaces that mate when the mold is joined. At the initial stage of the covering process, the core of the golf ball is supported centrally within the mold by retractable pins or the like so as to leave a space for molding a cover about the core. A thermoplastic cover material then is injected into the mold cavity in a horizontal plane from a primary supply through a plurality of edge gates that usually are evenly distributed near or around the parting line of the mold halves and the equator of the inner hemispherical surface of the golf ball. The retractable pins hold the core in place while the thermoplastic cover material fills the void between the core and the inside wall of the mold. Flow of thermoplastic cover material from each of the plurality of gates eventually joins to fill the void between the golf ball core and the mold. Once the void is nearly filled, but before the thermoplastic cover material has completely hardened, the centering pins holding the core in place retract so that the thermoplastic cover material may fill the voids left by the pins. The thermoplastic cover material then cools and hardens to form the cover.
For the last two decades, thermoplastic resin blends have been used extensively in injection molding processes as cover materials for golf balls. Over the years, such resin blends have been used to form very durable golf ball covers while also providing acceptable in-play characteristics such as spin rate, feel and initial velocity. Additional components conventionally added to cover compositions include ultraviolet light stabilizers and/or absorbers, optical brighteners, fluorescent pigments, dyes, processing aids and fillers. While modern day thermoplastic resin blend cover materials described above are satisfactory and in fact are more durable than previous cover materials, their use in multiple edge gate injection molding may result in increased occurrences in "flow front failure" along the knit lines of the cover.
Golf ball covers typically exhibit one of two types of failures: (1) "random failures"; or (2) "flow front failures." "Random failures" are flaws such as cuts, cracks or other fractures that appear anywhere on the outer surface of the cover of a golf ball after it has been struck with a club. Random failures may appear anywhere in the cover and either are the result of a random defect in the cover or occur due to mishitting the ball. "Flow front failures" are defects such as cracks, fractures or other surface defects which appear along knit line areas of the cover. Knit lines are seams along the cover that are formed where thermoplastic cover material intermixes from different sources during the formation of the golf ball cover. For example, when a golf ball cover is formed by compression molding, a knit line forms at the mold parting line along the equator of the ball where the thermoplastic resin material of the hemispherical cups flows together. The parting line flash is subsequently removed by cutting the excess material and/or buffing the ball. The "flow front" of such compression molded golf ball covers corresponds to the location of the parting line.
Similarly, when a golf ball cover is formed using a conventional retractable pin injection molding process with multiple edge gates to inject a thermoplastic resin blend into a mold, the thermoplastic resin blend from each gate has a flow front that eventually abuts cover material entering the mold from other gates. As such, there are a number of knit lines or flow fronts throughout a cover where cover material from various gates flows together as it fills the void between the golf ball core and the mold. Depending on the composition of the thermoplastic resin blend, the cover material tensile strength can be reduced by as much as 10% to 60% along the knit lines.
Because the cover is inherently weaker along the knit lines, the knit lines are more susceptible to flow front failures. As mentioned above, it is generally recognized in the golf ball industry that the occurrence of flow front failures is not the result of random flaws or imperfections along the seam of the ball or from mishitting the ball, but rather are a result of an incompatibility and/or incomplete mixing of the various components of the cover material and cooling that takes place at the flow front before the thermoplastic resin blend intermixes with cover material from different sources, i.e., from different gates. Thus, the use of a plurality of gates in an injection molding process generally results in the presence of knit lines.
Therefore, there exists a need for a method of making golf ball covers by an injecting molding process which does not result in the occurrence of knit lines, thereby increasing the durability of the golf ball cover and extending the useful life of the golf ball.
In addition to resulting in knit lines that may weaken the golf ball cover, conventional multiple edge gate injection molding also may not maintain balanced flow or uniform filling of thermoplastic resin blend cover material between the core and the inside wall of the mold, which may further weaken the golf ball cover. For example, non-uniform filling can cause the flow terminus of the cover material to not meet at the poles of the ball where trapped air and gasses typically are released through a vent. When the flow terminus is not at the poles of the mold, the trapped air and gasses cannot evacuate the cavity effectively, thereby further compromising knit line integrity and reducing the durability of the cover.
Actions taken to overcome unbalanced flow of cover material injected into a golf ball mold through a plurality of gates also may weaken the golf ball cover. There are two known causes of unbalanced flow in conventional multiple edge gate injection molding. First, the supply of cover material to each gate may not be identical to other gates in the mold, thereby causing unbalanced flow and pressure upon the core during the injection molding process. One remedy for this potential problem is for the flow length in the runners feeding each gate to be equal and for each gate to be dimensionally identical. Any dimensional discrepancies in these gates may cause unbalanced filling. Because dimensional discrepancies play such an important role in achieving balanced flow, conventional multiple edge gate injection molds may utilize an even number of gates, i.e., 4, 6, 8 or even 10 gates, in a two-prong secondary runner system where the dimensions of the prongs are virtually mirror images of each other. Furthermore, it is desirable to distribute the plurality of gates symmetrically about the mold in order to maintain balanced pressure and flow about the core. Dimple patterns of the ball, however, may require that the gates not be symmetrically spaced about the equator of the ball, which results in unbalanced, nonuniform filling of the mold.
Another way to address unbalanced fill caused by the location and geometry of the gates is to keep the retractable pins engaged with the core until the mold is nearly completely filled with cover material. In order to maintain reasonable concentricity between the cover and the core, however, a significant percentage of the core must be covered with material before the pins can be retracted. Thus, the cover material may fill the mold and surround at least one of the engaged pins centering the core within the mold. While the cover material is solidifying, yet still moldable, the pins can be retracted and the remaining holes are filled in.
Keeping the retractable pins engaged with the core while the cover material surrounds the pins, however, can lead to a second cause of unbalanced, nonuniform fill. Such contact with the retractable pins causes the cover material to immediately cool, which in turn slows the progression of cover material and causes the flow terminus to not meet at the poles of the ball. This results in poor venting and weakened knit lines.
Moreover, the number and arrangement of retractable pins primarily depends upon the dimple size and pattern of the golf ball because the faces of the retractable pins usually are shaped to form a dimple of the golf ball cover. Conventional dimple patterns are Icosahedron in shape, thereby dictating that there be five retractable pins at both poles of the mold to position the golf ball core during the injection process. As a result, the number and location of retractable pins for conventional multiple edge gate injection molding usually does not correspond to the number and location of gates. This disparity can cause the resin blend cover material to reach each of the retractable pins at different times, which in turn causes the flow terminus for the cover material to be located away from the poles of the mold where air and other gasses are vented.
Thus, while it is possible to counteract unbalanced, nonuniform flow that unavoidably results from use of conventional multiple edge gate injection molding, it is preferable to eliminate or reduce unbalanced, nonuniform flow.
FIGS. 1 and 2 illustrate the problems encountered and described above when utilizing multiple edge gates 10 for a golf ball mold. It is evident that cover material entering a mold through a plurality of gates causes discontinuous point source forces on the core of the golf ball that can result in unbalanced, nonuniform filling of the mold. These discontinuous point source forces must be carefully balanced in order to ensure that the core 12 of the golf ball maintains its centered position during the covering process.
FIG. 1 illustrates the formation of knit lines 14 as flow from any one gate 10 abuts with flow from a neighboring gate 10. As the mold is filled, knit lines 14 are formed between each gate 10 where the cover material abuts with an opposing flow source. The multiple knit lines of the cover ultimately intersect at the flow terminus of the cover material. While it is desirable that the flow terminus be located where trapped air and gasses are vented, i.e. at the poles of the mold cavity, it is difficult to control the location of the flow terminus with conventional multiple edge gate injection molding. Moreover, cover material injected into a golf ball mold through a plurality of gates can cause the mold cavity to fill unevenly. This uneven filling of the mold leads to cover material reaching each of the retractable pins 16 at different times, as shown in FIG. 2, which in turn further disrupts the filling of the mold.